Freeze-fracture and thin-section electron microscopy were used to describe the sites of attachment of 2 kinds of ectosymbiotic bacteria to a devescovinid flagellate from termites. In each case, surface specializations in both partners occur at the junctional complexes. Rod bacteria lie in pockets of the eukaryotic membrane which are coated by dense material and contain high densities of intramembrane particles. A double row of closely spaced particles circumscribes the edges of the pockets on the P face. The surface of the rod bacteria which is exposed to the external medium bears a thick glycocalyx and flagella. Fusiform bacteria are attached along ridges of the protozoon surface. Dense material underlies the ridges, and particles are aggregated on both P and E faces of the ridge membrane. The outer layer of the fusiform bacteria is grooved to match the host ridge. The bacterial-devescovinid junctions are considered to serve mainly as attachment sites, and the membrane specializations at the sites of contact are discussed in this respect. Control of junction assembly by the cortical bacteria is suggested by the parallel patterns of junctional replication and bacterial reproduction.